A variety of switches and other actuators have been developed for switching an electrical circuit in response to a mechanical overload condition. One such mechanical overload condition is an overpressure condition, where fluid pressure exceeds a predetermined threshold. Another such mechanical overload condition in an underpressure condition, where fluid pressure is less than a predetermined threshold. In response to the overpressure or underpressure condition, the switch interrupts one electrical circuit, completes a second electrical circuit, or both. This switching causes the associated circuit or circuits to act to correct the detected condition.
One prior art switch which trips in response to an overpressure condition uses a flat metal leaf, fixed at one end, the other end having one or more electrical contacts. Mechanical force is applied along the leaf. This force distorts the leaf so that it has two mechanically stable operating positions, one when the leaf is convex, the other when the leaf is concave. The detected pressure is applied to the leaf when the leaf is in the convex position. When the pressure exceeds a threshold, the leaf snaps from the convex position to the concave position and the switch trips, interrupting one circuit, completing another circuit, or both. The threshold is set by the physical design of the switch including the thickness and elasticity of the leaf and the applied distorting mechanical force. The switch further includes a reset button. When the reset button is pressed, an insulating portion of the reset switch engages the leaf and returns the leaf from the concave to the convex position.
One limitation of this prior art switch is the reduction in force applied by the leaf to the switch contact before the switch trips. The pressure applied to the leaf urges the contacts which complete the circuit apart in response to the overpressure condition. As the pressure increases, the force the leaf applies to close the contacts decreases until the pressure is sufficient to move the leaf from the convex position to the concave position.
This reduction in force creates an operational limitation for this prior art switch. Many such switches operate in environments where the switch is subject to conditions such as mechanical jarring or vibration. The reduced force applied by the leaf to the contacts immediately prior to tripping may permit jarring or vibration to trip the switch prematurely, causing inaccurate operation. In addition, the reduced force in the presence of jarring or vibration may cause contact chatter, i.e. the repeated high frequency opening and closing of the contacts without actual switch tripping. Chatter, in turn, may cause contact arcing which damages the contact surfaces. Due to contact arcing, the contact surfaces or contacts may need to be inspected and replaced more frequently, increasing the cost and likelihood of inaccurate operation.
Other switches have been developed which use an M-blade as a snap-acting mechanical switching element. The M-blade is fabricated with a split either between its outer legs or at its center. A bi-stable condition of the M-blade is created either by pulling its outer legs together or by spreading its center legs apart. A portion of the metal M-blade is fixed to a support with force applied to distort the legs of the M-blade. In this arrangement, the legs of the M-blade snap in an over-center fashion between a first mechanically stable position and a second mechanically stable position.
An important characteristic of the M-blade is that the force applied by the legs of the blade increases when approaching snap and remains high until slightly before the point where snap occurs. While the M-blade contact force decreases slightly prior to snap, the contact force is still considerably higher than switches in which the contact force steadily declines to zero just prior to snap. Thus, switches have been made utilizing M-blade mechanisms to provide improved performance in conditions where severe vibration or jarring may occur.
In addition to tripping in response to a detected condition, a switch must be resettable following correction of the overload condition to allow subsequent detection and tripping. Manual reset switches require actual intervention by the user or operator to depress the reset button or otherwise provide a reset actuation. However, the overload condition may occur or continue while the reset actuation is taking place. A trip free reset switch prevents the protective purpose of the switch from being defeated by holding down or jamming the reset button to continue providing the reset actuation.
The aforementioned prior art switch using a concave-convex leaf as a pressure-sensitive switching element lacks a convenient trip free reset capability. Resetting is more directly accomplished by depressing a reset button which engages the center portion of the leaf and moves the leaf from the concave position to the convex position, resetting the switch. However, holding down or jamming the reset button holds the leaf in the convex position, preventing tripping of the switch and allowing a potentially dangerous overload or overpressure condition to continue. No convenient, trip free, resettable switch has been implemented using an M-blade.
Accordingly, there is a need for a trip free, resettable switch which is immune to premature tripping as a result of vibration or jarring.